This invention relates to compressors in general and, in particular, to an improved screw-type compressor having a lubrication system for supplying lubricant to and draining it from shaft bearings, with each rotor shaft being surrounded, at a location between the shaft bearing and the working space of the compressor rotor, by an annular drain space for removing the escaping lubricant and gas, and by a packing which seals the shaft.
West German Offenlegungschrift No. 24 41 520 discloses a compressor in which the annular drain space forms a part of an expensive packing arrangement of great overall length, intended for taking unfiltered outside air into the working space of the compressor and preventing the lubricant and coolant from escaping from the working space along the shaft. For this purpose, the packing arrangement is composed of further annular spaces, in addition to the drain space, of which one is used as a pressure seal space, that is, pressurized from a compressed gas source, more particularly, from the compressor outlet. The annular drain space is open to the ambient. Thus, the gas passing from the pressure seal space into the drain space is lost. Therefore, such a packing arrangement is usable only if the compressor is enclosed in an airtight manner or if an air pressure shaft is available, because similar losses of another gas, undergoing compression in the compressor, for example, a coolant, would not be acceptable. Even with the compression of air, the losses through the pressure seal chamber must be minimized in order to avoid undue lowering of the efficiency of the compressor. The packing arrangement must, therefore, satisfy high sealing requirements. In practice, this would mean, particularly with higher working pressures of the compressor, the provision of a great overall length of the packing. Packings with a large length, however, require a larger spacing of the bearings which may result in inadmissible deflections and bending stresses in the rotor at relatively small loads.